If the field holds an array, then the $in operator
selects the documents whose field holds an array that contains
at least one element that matches a value in the specified array
(e.g. <value1>, <value2>, etc.)

Changed in version 2.6: MongoDB 2.6 removes the combinatorial limit for the $in
operator that exists for earlier versions of the operator.

This query selects all documents in the inventory
collection where the qty field value is either 5 or
15. Although you can express this query using the
$or operator, choose the $in operator rather
than the $or operator when performing equality checks on
the same field.